The present disclosure relates to mobile, wireless communication devices, examples of which include handheld, devices such as cellular telephones, personal digital assistants, wirelessly enabled notebook computers, and the like; and more particularly to controlling the emission of the radio frequency signals transmitted by such wireless communication devices to achieve compliance with governmental regulations regarding a specific absorption rate limit.
A wide variety of types of mobile, wireless communication devices are on the market for communicating voice, data, images, and other forms of information. The demand for smaller and thinner devices, present numerous challenges for the antenna design. The antennas must be designed to fit in a limited available space and support various operating characteristics. Because of the close proximity of the phone to the user, compliance with specific absorption rate (SAR) requirements can be a challenge. In FIG. 1 a wireless device 10 with an antenna 12 is shown as being used by a user 14. The antenna can be located internal or external to the device 10. When the device is held against the ear of the user 14, some of the transmitted radio frequency energy emitted from the antenna 12 is absorbed by the user's body, most notably the head 16. A measure of absorption of energy at a particular radio frequency per unit mass of tissue is specified as the Specific Absorption Rate (SAR). As will be appreciated, the SAR value depends heavily upon the location of the transmitting antennas with respect to the body and the intensity and the duration of the transmitted energy.
Government agencies, such as the Federal Communications Commission (FCC) in the United States of America, have adopted limits for safe exposure to radio frequency (RF) energy. For example, the FCC limit for exposure from cellular telephones is a SAR level of 1.6 watts per kilogram (1.6 W/kg), which is referred to as a specific absorption rate limit.
Voice and data transmissions may employ a communication protocol in which the transmissions occur in one millisecond transmission slots contained within a 20 millisecond frame. When transmitting data, it is desirable to utilize as many of transmission slots in each frame as possible in order to send the data quickly. However, the more of the frame that is used, the greater the RF energy that is emitted and thus the specified SAR limit may be exceeded by the data transmission.
As a consequence, in order to comply with the SAR limit, prior communication devices often transmitted with less than an optimal number of transmission slots in each frame and less that the desired signal intensity.